Various types of spinal column disorders are known and include scoliosis (abnormal curvature of the spine), kyphosis (backward curvature of the spine), spondylolisthesis (forward displacement of a lumbar vertebra), and other disorders such as ruptured or slipped discs, broken or fractured vertebrae, and the like. Patients who suffer from such conditions usually experience extreme and debilitating pain. In severe cases treatments for these conditions have used a technique known as fusion with spinal fixation which results in the surgical/mechanical immobilization of areas of the spine and the eventual fusion of the vertebrae in the regions treated. In less severe cases treatment comprises decompression of the affected nerve and fusion of the vertebrae involved. Such decompression is commonly used as a treatment for such conditions as degenerative disc disease, spondylolysis and spondylolisthesis.
Anterior interbody fusion techniques are used to achieve decompression of the affected region of the spine. In such procedures the intervertebral disc is removed and the vertebrae end plates on either side of the removed disc are fused together to stabilize the vertebral column. Such fusion techniques use various types of bone grafts to replace the disc. After a period of time the bone grafts and the adjacent vertebrae grow together and fuse.
To ensure proper growth and fusion it is necessary to apply a force or pressure to the surfaces to be fused thus keeping them in close contact and promoting bone growth. This pressure is of particular importance in the lumbar region of the vertebral column where the natural curvature of the spine results in the space between the anterior side of the vertebrae, or the anterior depth of the intervertebral disc, being greater than the space on the posterior side of the vertebrae. As a result of the curvature the load carried by the vertebral column, in the lumbar region is placed on the posterior side of the vertebra, making the spinal column unstable. To ensure fusion and to stabilize the vertebral column the load must be transferred to the anterior side of the vertebrae by mechanical/surgical techniques.
One such technique employs the use of an "H"-shaped element which fits between the spinous processes of adjacent vertebrae. Other such techniques employ ceramic blocks, which may be "H"-shaped. These devices are forced between adjacent spinous process surfaces thus forcing the spinous process apart and thereby closing the gap between the vertebrae on the anterior side of the vertebrae. Such fixation devices are usually used in conjunction with spinal fixation or wiring to place appropriate force on the anterior surface of the vertebrae. Such designs have the problem of the H-shaped element or ceramic blocks being able to slip out of place, thus requiring further surgery after the initial implantation. Additionally the wiring used in conjunction with the H-shaped element or ceramic block often requires piercing of the spinous process, to attach the wire to the vertebrae, which can lead to extensive trauma and damage to the bone of the vertebrae. In addition the wire can cut through the bone of the spinous processes, under extension load, due to the small "footprint" of the wire.
Other techniques have used compression plates which are screwed to the anterior surface of the vertebrae in conjunction with disc removal. Such devices have the disadvantage that the screws may pull or "cut out" of the cancellous bone of the vertebrae. Such treatment, as the attachment of the screws and their subsequent failure, leave the bone of the vertebrae damaged and weakened.
It is desirable that a fixation device be provided which facilitates secure "locking" of the fixation device to the vertebrae. It is also desirable that the device be non-damaging to the bone tissue to which it is attached.